In a general digital broadcasting system, because data are transmitted through limited frequency bandwidth, time dispersion effect can be occurred that pulse energy of a symbol is dispersed to a neighboring symbol pulse.
Also, in transmitted digital data, an Inter-symbol Interference (ISI) can be occurred due to multipath, frequency offset, and phase jitter, etc.
A repeater and a receiver of digital broadcasting use a channel equalizer, in particular, use a decision feedback equalizer updating a coefficient of a filter coefficient, in order to adaptively compensate channel effect.
FIG. 1 is a block diagram of a conventional decision feedback equalizer. As shown, the conventional decision feedback equalizer includes a Feed Forward Filter (FFF) 100 and a Feed Back Filter (FBF) 140 for removing Inter-symbol Interference (ISI), a symbol detector 120 for detecting a symbol from the filtered signal by comparing with a predetermined threshold, an error calculating unit 180 for calculating an error used for updating coefficients of filters, and a training sequence storing unit 160 for storing a predetermined training sequence of received data.
The training sequence is inserted into the digital broadcasting data according to predetermined interval and is used for reducing a decision error of an equalizer by estimating digital broadcasting channel characteristic.
Therefore, if the predetermined training sequence from the digital broadcasting transmitter is received, the conventional decision feedback equalizer opens an eye pattern which is a decision factor of performance and a reference for deciding an output signal, based on the training sequence stored at the training sequence storing unit 160.
The conventional decision feedback equalizer decides symbols from data having no training sequence if the eye pattern clearly opens. Because the FBF 140 removes the ISI occurred by a previously detected symbol, noise enhancement doesn't occurred, which often occurred in the linear equalizer.
However, because the conventional decision feedback equalizer uses a simple slicer as a symbol detector, it is much dependent on the eye pattern and if the eye pattern opens incorrectly, it is easy to detect symbols incorrectly.
If there is an error in symbol decision, the error is accumulatively spread over the whole equalizer through a feedback loop while passing the FBF 140. The error affects to the symbol decision and finally decreases the reliability of the decision feedback equalizer.
In the other side, there are many trials for reducing the decision error of the decision feedback equalizer in the data having no training sequence.
Representatively, a technology that uses a viterbi decoder having the decoding delay and allows the coefficient update of the equalizer to have the decoding delay, proposed in an article by G. Long, entitled “The LMS Algorithm with Delayed Coefficient Adaptation”, IEEE Trans. Acoust., Speech, Signal Processing, vol. ASSP-37, on October, 1989.
Also, a technology that uses the viterbi decoder and solves the decoding delay by adding periodical interleaver and de-interleaver, proposed in an article by M. V. Eyuboglu, entitled “Detection of Coded Modulation Signals on Linear, Severely Distorted Channels Using Decision-Feedback Noise Prediction with Interleaving”, IEEE trans. Commun., vol. COM-36, p. 401-409, on April, 1988 and in U.S. Pat. No. 4,833,693.
However, because these technologies use the viterbi decoder whose delay value is (TBD-1) as a symbol detector, there are drawbacks that not only additional devices are required for removing the decoding delay but also 5 times memories than used in decoding are required in order to have sufficient performance of next parts of the equalizer.
Also, because American standard terrestrial digital broadcasting system uses twelve TCM encoders, a real decoding delay value of the decision feedback equalizer is 12× (TBD-1).
The decision feedback equalizer having a large decoding delay is not suitable for the digital broadcasting system requiring a quick decision of the received signals.
Therefore, the decision feedback equalizer that detects symbols correctly from data having no training sequence and has small decoding delay is strongly required.